I noticed some further strange behavior even after I disconnected the Reset pin (also tried with it connected to +5V).

The display is connected to pins 22-34.I upload a sample sketch from GLCD called BigNums. It works as expected. After some time I try to upload Blink basic sample and it hangs (again RX LED lights up for a while, then turns off, and then nothing happens).After I kill avrdude.exe and try to re-upload Blink it may either succeed or fail. However if I disconnect any of pins the display is connected to (22-34) the upload will always succeed.

What can be the reason of this?

EDIT:Looks like this is happening even if the display is not connected. Sometimes the upload will hang and disconnecting any pin (even if it is not connected with the display) seems to resolve the issue...

EDIT:Looks like this is happening even if the display is not connected. Sometimes the upload will hang and disconnecting any pin (even if it is not connected with the display) seems to resolve the issue...

"Disconnecting any pin" ? Not sure what you mean by this. Are there other things hooked up to the mega?Perhaps there is too much load on the power supply or the AVR pins.

Sorry for being unclear... I have made additional tests and my previous statement (the EDIT one) doesn't seem to be true.I have my display connected to pins 22-34 and a 4x4 keypad connected to pins 36-43.Then I upload BigNums sample.After that I try to upload Blink sample. If it hangs, I disconnect any one of the pins 22-34 and then re-uploading seems to work. After that I connect that pin again.

I have my display connected to pins 22-34 and a 4x4 keypad connected to pins 36-43.

This the first mention of additional hardware (keypad) being attached.So there are other things hooked up in addition to the glcd.This is why I asked about other things being hooked up to the mega.

I'm still guessing that this is due totoo much load on the power supply or the AVR pins.

This can be due to a variety of things from simple miswiring to trying to pulltoo much power from an Arduino pin or the main power supply.

Do you have a voltmeter?If so check the VCC voltage and see if it is low particularly when the upload is failing.(this probably indicates too large of current draw somewhere, possibly due to miswiring or a short)

First I would check to see if things work with only the glcd hooked up.

Then slowly start to add the other components to see what causes the failure.

ok,wait a minute, the keypad is on pins 36-43?The glcd library is using pins 36 and 37 so you have pin collision, which has the potential to create a short.This is very bad. Make sure you are not sharing any pins between the glcd library and the keypad.

This the first mention of additional hardware (keypad) being attached.So there are other things hooked up in addition to the glcd.This is why I asked about other things being hooked up to the mega.

Sorry for not telling this. I had some things connected to my Mega and while investigating the problem disconnected everything, but forgot about the keypad.

Quote

Do you have a voltmeter?If so check the VCC voltage and see if it is low particularly when the upload is failing.

Am I right that I should check the voltage between +5V and GND?If so, I don't see changes when uploading code against when the code is normally run.When the LCD sample is on I get 4.95V.When Blink sample is run I get 4.80-4.86V. Don't know why it drops.Without the display I get 4.99V.However upload hanging seems random... It may occur when GLCD is run and I try to switch to Blink while voltage is still 4.95V. It may occur when I already run Blink (after BigNums) and try to re-upload Blink (at 4.85V). It may stop occurring in the last case with the same voltage. BTW disconnecting a single pin from the display (say, pin 22) doesn't seem to change voltage, though it seems to help to re-upload sketch after upload hangs and avrdude.exe is killed (but this may be just my thoughts, however it always helped me).

Quote

The glcd library is using pins 36 and 37 so you have pin collision, which has the potential to create a short.This is very bad. Make sure you are not sharing any pins between the glcd library and the keypad.

I wanted to gather all the pins used by the display and the keypad together, so I changed some constants. Here's my modified version of ks0108_Mega.h (I hope this is the only file I need to modify):

/* * ks0108_Mega.h - User specific configuration for Arduino GLCD library * * Use this file to set io pins * * The configuration below uses a single port for data * and has the same wiring as the mega config from the previous ks0108 library **/

// Reset Bit - uncomment the next line if reset is connected to an output pin//#define glcdRES 30 // Reset Bit

#endif //GLCD_PIN_CONFIG_H

EDIT:Looks like the problem might be connected with the keypad and constants reassignment. Whenever I disconnect the keypad completely I can't reproduce the hang. Connecting pins 36 and 37 of the keypad still produces no problem (I tried multiple re-uploads). But after connecting pin 38 along with 36 and 37 I got a hang. So there must be something else I need to change in GLCD in order to use reassigned pins...

EDIT2:I think the problem is with pin 38. If I connect the keypad without it, everything uploads fine. With it I get hangs. Does this pin have a special meaning?

Looks like the problem might be connected with the keypad and constants reassignment. Whenever I disconnect the keypad completely I can't reproduce the hang. Connecting pins 36 and 37 of the keypad still produces no problem (I tried multiple re-uploads). But after connecting ping 38 along with 36 and 37 I got a hang. So there must be something else I need to change in GLCD in order to use reassigned pins...

I'm not sure why the glcd library pins need to be changed, but what you have seems to be ok and should work.There is nothing else to change for the glcd library to change the pins used.

Do you have a schematic of everything that is hooked up to the mega?

I'm still guessing that there is a loading issue of either the power supply or an AVR pin(s).

Perhaps the column/row lines on the keypad are mixed up or not hooked up correctly to the mega pins which is creating a shorteither during scanning or when a button is pressed.(I've seen this before)

What library is the keypad using?Does it scan the keypad using interrupts?

Can you attached the keypad and read the keys properly without the glcd attached?(as a test maybe do something like read the keys and send output to the serial port)

Bill, thank you very much for your help! However it looks like the problem is unrelated to both the display and the keypad.

I disconnected everything from the Mega. Blink sample uploaded well.Then I put a wire into pin 38 without connecting the other side to anywhere. Upload started hanging.Then I connected that wire with GND. Upload started working again.

Could you tell me, what might be so special about pin 38?Other pins don't seem to provoke such a problem.